1. Field of the Invention
The present invention generally relates to memory structures and, more particularly, to memory structures for use in E.sup.2 PROMs (Electrically Erasable Programmable Read Only Memories).
2. Description of Related Art
In the microelectronic arts, memory devices can be broadly characterized as either random access memories (RAMs) or read-only memories (ROMs). RAMs can be described as read/write memory devices whose contents may be altered at will and which can be randomly addressed by a two-dimensional addressing scheme (e.g., select and data lines). Also, RAMs have the characteristic of being volatile; that is, all information stored in a RAM is lost when power is removed. By way of contrast, ROMs have the characteristic of being non-volatile; that is, information stored in a ROM is not lost when power is removed.
Two examples of ROMs are electrically programmable ROMs (EPROMs) and electrically erasable PROMs (EEPROMs or E.sup.2 PROMs). EPROMs have the characteristic that they are field programmable. Data is written into EPROMs electrically, but a strong source of ultraviolet (UV) light is required to erase data. Typically, erasure of an EPROM requires removing it from its host circuit board. E.sup.2 PROMs are characterized by the fact that they do not require an ultraviolet light source for erasure and, consequently, do not need to be removed from a host circuit board for reprogramming. Typically, EPROMs and E.sup.2 PROMs are based upon metal-oxide-semiconductor field effect transistor (MOSFET) technology.